Semi-submersible offshore vessels used for oil production and for accomodation mainly comprise a work platform, carried by means of four or more columns resting on underwater hulls. The columns can be damaged during a collision with another vessel, or some large floating object, like an iceberg. As the columns, at least partly, are used for vertical communication, there will be a loss of displacing capacity if a column become water filled. There is then an apparent risk that the vessel will capsize, or at least will heel over so much, that a rescue operation will be difficult to carry out.
The object of the present invention is to propose an arrangement, which generally will improve the stability of the vessel and which gives a good margin of safety from capsizing in an event of disaster. The invention facilities rebuilding of old vessels.